1. Field of the Invention
The invention relates to a delay circuit suitable for delaying differential signals, comprising a first and a second switching transistor, each of which comprises a control electrode and a first and a second main electrode, a first and a second input of the delay circuit being coupled to the control electrode of the first switching transistor and the second switching transistor, respectively, the first main electrode of the first and the second switching transistors being coupled to one another and, via a first current source, to a first power supply terminal, the second main electrode of the first and the second switching transistors being coupled to a first and a second output, respectively, of the delay circuit, the first and second outputs being coupled to a second power supply terminal via respective first load circuits. The invention also relates to a cascade of such delay circuits.
2. Description of the Related Art
A delay circuit of this kind is known from U.S. Pat. No. 5,146,121. The first and the second switching transistors control the connection between the current source and the load circuits. Depending on the difference between the voltages on the inputs, the current flows from the current source through the first or through the second switching transistor to the first or to the second output, respectively, and to the associated load circuit. Thus, the capacitance present on the output is alternately charged or discharged, depending on the difference.
For example, when the first switching transistor is turned on, the voltage on the first output decreases due to the discharging until the current source and the first switching transistor are saturated. The voltage on the second output increases until the associated load circuit is saturated.
When the input voltage changes so that the second switching transistor is turned on and the first switching transistor is turned off, the voltage on the second output decreases and the voltage on the first output increases. After a delay time on the outputs, a voltage level is thus reached which suffices to switch over other circuits. The rise and fall speed, and hence the delay time, can be adjusted by means of the current through the current source.
However, the switching time is also dependent on the initial voltage at the instant of switching over of the input voltage. When the switching transistor ceases to be turned on, the initial voltage is dependent notably on the saturation properties of the switching transistors and on the current source. Thus, the initial voltage is dependent on a spread in the parameters of these switching transistors. Consequently, the delay time is not reproducible very well.